1. Field of the Invention
This invention relates in general to the field of cellular telecommunications, and more particularly to an echo path compensation technique for use in an acoustic echo cancellation mechanism.
2. Description of the Related Art
Virtually all present day two-way communication devices, such as cell phones and the like, employ some forms of acoustic echo cancellation techniques and mechanisms therein to preclude unwanted echo from being transmitted back to a calling party. Particularly when these devices are used in a loudspeaker mode, the volume of their speaker is turned up so loudly that sound intended only for the receiving party is picked up by the microphone of the receiving device and is transmitted back to the calling party. This phenomena is known as near end acoustic echo and it is desirable to detect and cancel it out because optimally the only sound a calling party should hear coming from his/her speaker is that of the receiving party, not an echo of his/her voice.
Near end acoustic echo cancellation techniques abound, but most rely predominantly on using linear adaptive filters to dynamically and recursively model an echo path, that is the electro-mechanical-acoustic path which a received signal propagates when it is played out of the loudspeaker of a device and enters back in through the device's microphone. Ideally, the echo signal is filtered out and only sound produced by the near end party is allowed to be transmitted back to the far end party.
However, as one skilled in the art will appreciate, an adaptive linear filter is most effective when it is employed to model a system component (i.e., the echo path) that is linear, and there are several elements in the echo patch of any communication device that are not linear such as the speaker and microphone themselves, battery powered amplifiers, etc. Hence, to provide for acoustic echo cancellation by employing an adaptive linear filter exclusively results in residual echo that is transmitted back to the far end party. This is undesirable.
In U.S. Patent Application Publication US20050249349 Derkx et al. propose an echo canceller which has dedicated non stationary echo canceling properties comprising an adaptive filter followed by a residual echo processor that includes a dedicated non stationary echo canceller. Such a technique, while improving upon that which had theretofore been provided, deals only with residual echo from a stochastic systems point of view and thus does not consider known non-linear effects of its host platform.
In U.S. Patent Application Publication US20100189274, Thaden et al. propose a method suitable for coping with non-linear echo paths during acoustic echo cancellation in speakerphones. The method combines a linear adaptive filter and a post-processor together with a multiple microphone approach using beam forming which separately removes the non-linear part of the echo. The approach, which utilizes generalized side lobe cancellation principles to deal with residual non-linear echo components, requires the addition of multiple microphones and multiple beam forming units, thus significantly adding to the overall cost of a communication device.
Therefore, what is needed is a near end acoustic echo cancellation apparatus and method that compensates for non-linear elements within an echo path in a communication device, without the substantial cost of additional components such as microphones.
Additionally, what is needed is a acoustic echo canceller that utilizes knowledge of non-linear components in an echo path to pre-distort a received signal in amplitude prior to adaptive linear filtering.
Furthermore, what is needed is an apparatus and method for acoustic echo cancellation that compensates for phase misalignment between a microphone input signal and the output of an adaptive echo cancellation filter.